5.4 The Discovery of Auxin

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Del curso dictado por Tel Aviv University

Understanding Plants - Part I: What a Plant Knows

472 calificaciones

Tel Aviv University

Understanding Plants - Part I: What a Plant Knows

472 calificaciones

For centuries we have collectively marveled at plant diversity and form—from Charles Darwin’s early fascination with stems and flowers to Seymour Krelborn’s distorted doting in Little Shop of Horrors. This course intends to present an intriguing and scientifically valid look at how plants themselves experience the world—from the colors they see to the sensations they feel. Highlighting the latest research in genetics and more, we will delve into the inner lives of plants and draw parallels with the human senses to reveal that we have much more in common with sunflowers and oak trees than we may realize. We’ll learn how plants know up from down, how they know when a neighbor has been infested by a group of hungry beetles, and whether they appreciate the music you’ve been playing for them or if they’re just deaf to the sounds around them. We’ll explore definitions of memory and consciousness as they relate to plants in asking whether we can say that plants might even be aware of their surroundings. This highly interdisciplinary course meshes historical studies with cutting edge modern research and will be relevant to all humans who seek their place in nature. This class has three main goals: 1. To introduce you to basic plant biology by exploring plant senses (sight, smell, hearing, touch, taste, balance). 2. To introduce you to biological research and the scientific method. 3. To get the student to question life in general and what defines us as humans. Once you've taken this course, if you are interested in a more in-depth study of plants, check out my follow-up course, Fundamentals of Plant Biology (https://www.coursera.org/learn/plant-biology/home/welcome). In order to receive academic credit for this course you must successfully pass the academic exam on campus. For information on how to register for the academic exam – https://tauonline.tau.ac.il/registration Additionally, you can apply to certain degrees using the grades you received on the courses. Read more on this here – https://go.tau.ac.il/b.a/mooc-acceptance Teachers interested in teaching this course in their class rooms are invited to explore our Academic High school program here – https://tauonline.tau.ac.il/online-highschool

De la lección

How a Plant Knows Where it is?

This week we continue our systematic review of a plant's sensory systems by exploring the 6th sense - proprioception. We start with an overview of the proprioceptive system that allows us to keep our balance and to know where are body parts are in space. Theמ we will explore how plants know up from down, using both experiments from a few hundred years ago, and experiments conducted on the space station. We'll go over the structure of roots more in detail in order to understand where the cells are that sense gravity. We'll revisit phototropism, and learn what the chemical signal is in plants that allows them to respond to light and gravity. And lastly, we'll learn what makes a plant dance.

5.1 How a Plant Keeps its Balance4:56

5.2 Plant Response to Gravity3:55

5.3 Gravitropism in Roots10:08

5.4 The Discovery of Auxin5:31

5.5 Auxin Transport3:35

5.6 Auxin and Root Initiation8:19

5.7 Sensing Gravity6:50

5.8 Circumnutation7:26

Conoce a los instructores

Professor Daniel Chamovitz, Ph.D.
Dean, George S. Wise Faculty of Life Sciences
Director, Manna Center for Plant Biosciences

Darwin described at least two types of movements in plants.

In the second lecture, we talked about phototropism, plants

bending towards the light, and here we've learned about gravitropism.

Plants bending either towards gravity or away from gravity.

But we haven't talked about what's causing the bending.

And if you remember, Darwin concluded

from his phototropism experiments that there was a

signal moving from the tip to the base.

Or as he wrote in his book, we must

therefore conclude that when seedlings are freely exposed to lateral

light, some influence is transmitted from the upper to

the lower part, causing the latter to bend, an influence.

What is this influence?

What is this signal?

Well, Darwin didn't know, which is why he called it an influence.

But in the 140 years, 130 years since Darwin did his experiments.

We know what this influence is.

So how is this influence discovered?

So, the first experiments that we studied, we're now we're going back

to talk a bit about phototropism because this will help us understand gravitropism.

Where Darwin's

experiments where we covered the tip of the plant with

the light proof cap and we saw that it didn't bent.

And that's where we came to his conclusion about the influence.

Darwin's results were the basis for further experiments at the

beginning of the 20th century by the Dutch scientist Peter Boysen-Jensen.

And what, Boysen-Jensen did, was he cut off the tip of

the seedling. And then put it back on the plant.

Now when he cut off the tip and put it back on

the plant, the plant still had the ability to bend towards the light.

So he then did the following experiment. He cut off the tip.

And then put it back on the plant, but before he put it back, you

put a slice of gelatin on the stalk, and then put the plant tip back

on it.

In the second plant, he put a piece of mica,

a piece of glass, between the stalk and the tip.

In the first case, when the tip and the stalk are separated

by a piece of gelatin, the plant still retained the ability to

bend, whereas when it was separated, the tip was separated from stark

by piece of glass, it lost the ability to bend to light.

It also lost the ability

to respond to light. It lost the ability to do phototropism.

So what's the conclusion from this experiment?

The conclusion is that the tip of the plant is generating a soluble signal

that is somehow traveling down the plant, influencing it to

bend. So Darwin's influnce is a soluable signal.

But the question then of course is what is the soluble signal?

And the soluble signal was isolated in 1926 by another

scientist named Went.

Now his hypothesis was that the gelatin or what we actually call agar contains a

chemical produced by the tip which stimulates growth further down the stock.

So to test the hypothesis, he did the following experiment.

He took the tip, and then put it isolated on top of the agar.

Let it stand for a little bit, and then

placed the agar directly on the stalk of the

plant, a plant that had had its tip cut

off, a decapitated plant, but without the tip itself.

What he saw was that when

he put the agar squarely on top of the stalk, the stalk then resumed growing up.

Again this is a stalk that itself does not have a tip.

But if you just put the agar itself that had never, add a

slice of agar that had never exposed to the tip of the plant.

There was no, there was no influence on the stalk, it did not grow.

What this is showing is that it wasn't the agar itself that

contained a chemical that was causing the plant to grow.

But rather that the agar that had been exposed to the tip.

Contained some type of chemical which was causing the plant, the stalk, to elongate.

Then if he put the agar on only half the stalk, so for example, let's

say this is the stalk and I'm only putting the agar here on one side.

What would happen was that

the plant would then bend towards the other direction.

If I were to put the agar again, on this

side, then the plant would bend to the opposite direction.

In other words, somehow or another, the

agar was causing the plant to elongate, and

if it was only on one side of the stalk, only one side is elongating.

And that's sort of like what was happening in phototropism.

He isolated from this agar a chemical, which was then called auxin, which is

taken from the Greek word to increase, in other word, to increase its length.

So auxin is the first plant hormone.

Its a phytohormone to be isolated.

And one of the roles of auxin is to stimulate growth.

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